[zbailey]

I believe a hosted zone simply corresponds to a single domain you want to host the DNS for.

If you've ever ran your own DNS server this would be equivalent to a "zone file": http://en.wikipedia.org/wiki/Zone_file

[dabeeeenster]

If I wanted to point

www.domain.com

and

svn.domain.com

to separate IP's, would both of these count as a single domain or would I need to pay for two?

[dangrossman]

Those would be part of one zone, the domain.com namespace.